I'm currently experiencing some test failures here, which I think may be related to `local` mode.  It appears that `updateEpoch` is being called on the `MapOutputTrackerMaster`, which is probably happening because that same MapOutputTracker is being passed to a local Executor.  The assertion was based on the idea that the workers' epochs should always be less than or equal to the master's epoch.  In this case, we'd expect them to be exactly equal because the epoch is incremented on the master, and that epoch is used to create the tasks.  Tomorrow, I'll try to debug this locally.